1. Field of the Invention
The present invention relates generally to photo-keratometry, and more particularly, to improvements in the art of photo-keratometry wherein a diffuse reflection of a projected, illuminated surface or other target is formed on the cornea. This diffuse reflection is analyzed to determine the surface contour of the eye. The present invention specifically relates to devices that are used for clinical keratometry
2. Description of Related Art
The instruct that is in most common use for central-optical-zone shape measurement, in ophthalmic clinical practice, is the EyeSys topographer. Several companies offer similar devices, from outright copies to apparatus redesigned using a similar principle of operation. These devices require the user to operate one or more controls. These controls are employed to bring Mire images that are reflected from the tear film on the surface of the eye, simultaneously into focus and alignment.
Most prior art devices require that the tear film on the eye surface be intact since reflection from this film is central to the measurement technique. The tear film is typically not constant in thickness and, in some cases, may not be present at all. This variable thickness or outright absence of the tear film prevents meaningful measurements of the corneal shape. In addition, if the corneal surface is not smooth as in the case of corneal transplants, the reflections are ambiguous and do not permit meaningful determination of the surface shape.
Representative of video keratometers is the EyeSys System 2000. A placido (bulls-eye target) is illuminated by an internal lamp system, and a video image of the reflex is examined by conventional video analysis means. A second generation of these instruments provides for analysis of both corneal surface contour and shape/location mapping of more posterior elements such as corneal thickness, anterior chamber depth and lens geometry. Representative of these is the Orbtek Orbscan. In this instrument, a sequence of optical slit images are formed by conventional projection means and the resulting Tyndall images are used for determination of the mapping information. Requirement for a sequence of images requires that the subject be able to fixate perfectly for a long period of time without either head or eye movement and the large amount of data collected requires extensive calculations which require expensive computers for data analysis.
Prior art systems, however, are costly, complex, and slow, and modifications, if required, are difficult to implement. Thus, it follows that another system for characterizing the eye must be employed in order to produce a functional instrument within the speed and cost constraints required for commercial viability. Additionally, a practical system must be suitable for operation by unsophisticated users. These above-listed factors, as well as other criteria, require a departure from the traditional techniques for keratometry and image analysis.
In recent times, keratometers have been more commonly used in both clinical and surgical ophthalmology and optometry. In spite of the number of systems in use today, universal satisfaction with the results of these systems have not been obtained.
None of the currently available video keratometers, however, are designed to overcome most of these above-mentioned problems. Additionally, the prior art systems exhibit a wide range of error and are inconvenient to operate.
Thus, there remains a need for an apparatus and method for measuring the optical components of the eye that avoids most, if not all, the foregoing problems.